Joke around with the attractive customers. I was a cashier, and usually the hot chicks that would come in once, would come in again. If that's where they shop, then that's where they shop! I used to tease the hot ones, never number close or compliment them in the register though. Your goal is to build rapport so she knows who you are, then see her around outside of work, and boom. Examples of what I used to do:
(Chick hands me a 20 dollar bill)
I give it right back to her and with a serious face, say, "We don't accept counterfeit twenties." I haven't failed with this one; always gets a laugh. Afterwards, obviously, I laugh with them.
Talk about what they buy, unless it's a personal product or embarassing. Use good judgement!
Keep it light, be cool.